Various physiological functions of electrolytes are widely known. It is necessary, however, to incorporate at least a specific concentration of an electrolyte in order to prepare pharmaceutical compositions that are capable of effectively exhibiting the functions provided by electrolytes. Since electrolytes have a characteristic of decreasing the strength of membranes formed at oil/water interfaces, it is known that when an emulsion composition, particularly an O/W emulsion composition, containing an electrolyte is prepared, emulsion particles agglomerate and are likely to cause a separation of water and oil phases, oil floating, etc. Moreover, the emulsion itself sometimes degenerates over time due to the influence of the electrolyte, resulting changes in color and smell.
When preparing emulsion compositions containing an electrolyte, therefore, various pharmaceutical techniques are employed to improve the stability of the emulsion.
Although polyglycerin fatty acid esters are emulsifiers heretofore known to be highly safe and excellent in usability and functionality, the emulsifying ability thereof is relatively low. Therefore, when a large number of electrolytes are used, it is difficult to prepare a stable emulsion composition from the polyglycerin fatty acid ester. Recently, a nonaqueous emulsion method was proposed as a method for preparing a stable emulsion using a polyglycerin fatty acid ester (Japanese Unexamined Patent Application No. 37040/1981). Even when this method is employed, phenomena such as oil floating and the like sometimes occur as the concentration of the electrolyte increases and the emulsifying ability decreases. Further, an acrylic acid-alkyl methacrylate copolymer has recently drawn attention as an excellent high-molecular emulsifier for an O/W emulsion due to properties such as the ability to provide a stable emulsion by a small amount; the ability to be used regardless of the type of oil, thereby making it versatile; gentleness to humans and the environment; an excellent feel when applied to the skin and hair related to a lack of tackiness; etc. However, it is also reported that the viscosity and the emulsion stability are decreased when the acrylic acid-alkyl methacrylate copolymer is used with ionic components. (“Function and Application of High-Molecular Emulsifier for O/W Emulsion” FRAGRANCE JOURNAL 1998-8 pp. 79-83)